A biofuel cell is similar to a traditional polymer electrolyte membrane (“PEM”) fuel cell in that it consists of a cathode and anode generally separated by some sort of barrier or salt bridge, such as a polymer electrolyte membrane. However, biofuel cells differ from the traditional fuel cell by the material used to catalyze the electrochemical reaction. Rather than using precious metals as catalysts, biofuel cells rely on biological molecules such as enzymes to carry out the reaction. Early biofuel cell technology employed metabolic pathways of whole microorganisms, an approach which provided impractical power density outputs due to low volumetric catalytic activity of the whole organism. Enzyme isolation techniques spurred advancement in biofuel cell applications by increasing volumetric activity and catalytic capacity. Isolated enzyme biofuel cells yield increased power density output by overcoming interferences associated with cellular membrane impedance with electron transfer and lack of fuel consuming microbial growth.
Although enzymes are highly efficient catalysts, there have been problems incorporating them into fuel cells. Early enzyme-based fuel cells contained enzymes in solution rather than immobilized on the electrode surface. Enzymes in solutions are only stable for days, whereas immobilized enzymes can be stable for months. One of the main obstacles of enzyme-based biofuel cells has been to immobilize the enzyme in a membrane at the electrode surface that will extend the lifetime of the enzyme and form a mechanically and chemically stable layer, while not forming a capacitive region at the electrode surface. In most H2/O2 fuel cells, the binder that holds the catalyst at the electrode surface is Nafion®. Nafion® is an enzyme immobilization material that has excellent properties as an ion conductor. However, Nafion® has not been successful at immobilizing enzymes at the surface of biofuel cell electrodes because Nafion® forms an acidic membrane that decreases the lifetime and activity of the enzyme.
In addition to these challenges, there is also a desire to reduce the geometric scale of biofuel cells. Along these lines, biofuel cells to date have relied on some sort of physical barrier to separate the anode and cathode, but there is a persistent desire to construct a biofuel cell without such materials to reduce the size of the fuel cell. Such a development would advantageously allow for smaller biofuel cells, reduce raw material costs, simplify the method of construction, and eliminate problems due to fouling or damage of the electrode. In addition to barriers, the size of biofuel cells is limited by the method of forming the electrodes. Currently, electrodes are formed using carbon cloth or carbon paper with typical dimensions of 100 μm thick and 1 mm wide. U.S. patent application Ser. No. 10/617,452 describes such electrodes. A method of producing smaller electrodes would allow for the use of biofuel cells in a variety of micro scale applications.
A further challenge to improving biofuel cell performance is developing ways to increase biofuel cell power density. Currently, the biofuel cell's current density is limited by the diffusion of the fuel fluid to the electrode surface. It would be desirable to improve the biofuel cell's current density by increasing the transport efficiency of the electrodes. Since power is equivalent to the current density multiplied by the voltage, an increase in the biofuel cell's current density will yield a significant increase in the overall power density.
Further, another major problem with biofuel cell development has been the ability to easily form fuel cell stacks. A fuel cell stack is several individual fuel cells that are wired in series to increase the overall voltage of the cell. Particularly, conventional fuel cell stacks are limited dimensionally because of the need for bipolar plates to separate the individual fuel cells. This has made it impossible to meet the space constraints of micro applications. The ability to form fuel cell stacks with micro-dimensions would yield greater power density from smaller sources for various micro-scale applications.
Finally, the inability to form complex or irregularly shaped electrodes has hindered biofuel cell development. Traditional electrode formation techniques using the previously mentioned traditional electrode materials produce an electrode with flat topography. Since current capability is proportional to the electrode's surface area, a flat electrode yields the minimum current capability for given length and width dimensions. If there existed a method of producing electrodes with an irregular topography, however, higher current capabilities could be achieved as compared to similarly sized electrodes produced by conventional techniques.
With the above concerns and challenges in mind, a microfabricated fluidic approach is a possible way to develop a biofuel cell that will address each shortcoming of the current state of biofuel cell technology.